This paper deals with the role of science in large scale ecosystem projects, in particular bioregional assessment, in the United States and Europe. Bioregional assessment is a relatively new approach to ecosystem management that involves a scientific assessment of a so-called bioregion - an area that is demarcated by ecological instead of political boundaries. Scientists play an important role in this approach which is closely related to policy. The goal of this paper, therefore, is to examine new roles of science in natural resource policy and to study the reflection of these roles in bioregional assessment or similar approaches in the U.S. and Europe. The author developed her research design in accordance with grounded theory, which is a methodological approach to generate theory from qualitative data. The research was based on case studies that were used to develop general categories describing the role of science in ecosystem projects. The cases were examined by interviewing scientists as experts who had participated in the projects or activities ...